Understanding the Effect of the Human Body on RF Signal Propagation

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A person's body contains a large amount of water which is an excellent absorber of microwave frequency radiation. There are regulatory guidelines for maximum energy exposure. In almost all cases, people who are at least 2 feet away from a typical 802.11 Wi-Fi access point (with no more than a 5 dBi antenna) are probably within the recommended energy exposure limits.

Understanding the human body's effect on RF wave propagation is complicated by the fact that the body consists of components that each offer different degrees, and in some cases different types, of RF interaction. While it should be immediately evident that the liquid nature of most body structures offers a degree of RF attenuation it's not as immediately obvious that the skeletal structure introduces wave diffraction and refraction at certain frequencies.

One may recall that an electromagnetic wave (including light) experiences a refraction (bending) effect when passing between mediums with different densities. The spear fisherman standing in shallow water sees the fish at a location that's offset from the real position because the light refracts as it passes from the water to the air. This effect also occurs when an RF signal enters a human body. Not only does the body absorb some of the signal, it also refracts the signal.

Detailed research has been conducted concerning the interaction between human bodies and the electromagnetic spectrum. In general, when the wavelength of a signal is significantly larger than the cross-section of the human body being penetrated there is very little effect on the signal. These wavelengths occur at frequencies below 4 MHz. Above 4 MHz the absorption of RF energy increases and the human body may be considered to be essentially opaque. Above roughly 1 GHz the dielectric properties of the human body begin to introduce a scattering effect on the RF signal.

One may conceptualize this change from opaque to scattering by realizing that if the frequency of the electromagnetic wave continues to increase, and the wavelength therefore becomes shorter, and shorter, the human body eventually becomes a reflector of the signal (which is now in the frequency range of visible light), and you see the light reflected from the person and you say, “Hi Bob!”

When Connect802 creates an RF model and simulates signal coverage in a hospital, the patient's body is considered to be radio-opaque to the 600 MHz WMTS transmitter. However, as shown in the picture to the right, it is reasonable to assume that signal from the transmitter will be able to pass downward through the mattress and reach the WMTS receiving antenna as a result of natural signal reflection in the room.

Consequently, the WMTS patient-attached transmitter's location, relative to the patient, is not considered a factor in the design of the system. In the event that a patient is able to position themselves such that the signal is obstructed, the WMTS system will alert the nurses station and the transmitter can be repositioned.

The signal from the WMTS transmitter is able to penetrate downward through bedding material even when obstructed by the patient's body.